A REVIEW ON THE MATERIALS SCIENCE AND DEVICE PHYSICS OF SEMITRANSPARENT ORGANIC PHOTOVOLTAICS

In this review, the current state of materials science and the device physics of semitransparent organic solar cells is summarized. Relevant synthetic strategies to narrow the band gap of organic semiconducting molecules are outlined, and recent developments in the polymer donor and near-infrared absorbing acceptor materials are discussed. Next, an overview of transparent electrodes is given, including oxides, multi-stacks, thin metal, and solution processed electrodes, as well as considerations that are unique to ST-OPVs. The remainder of this review focuses on the device engineering of ST-OPVs. The figures of merit and the theoretical limitations of ST-OPVs are covered, as well as strategies to improve the light utilization efficiency. Lastly, the importance of creating an in-depth understanding of the device physics of ST-OPVs is emphasized and the existing works that answer fundamental questions about the inherent changes in the optoelectronic processes in transparent devices are presented in a condensed way. This last part outlines the changes that are unique for devices with increased transparency and the resulting implications, serving as a point of reference for the systematic development of next-generation ST-OPVs

DETERMINATION OF THE CHARGE CARRIER DENSITY IN ORGANIC SOLAR CELLS: A TUTORIAL

The increase in the performance of organic solar cells observed over the past few years has reinvigorated the search for a deeper understanding of the loss and extraction processes in this class of device. A detailed knowledge of the density of free charge carriers under different operating conditions and illumination intensities is a prerequisite to quantify the recombination and extraction dynamics. Differential charging techniques are a promising approach to experimentally obtain the charge carrier density under the aforementioned conditions. In particular, the combination of transient photovoltage and photocurrent as well as impedance and capacitance spectroscopy have been successfully used in past studies to determine the charge carrier density of organic solar cells. In this Tutorial, these experimental techniques will be discussed in detail, highlighting fundamental principles, practical considerations, necessary corrections, advantages, drawbacks, and ultimately their limitations. Relevant references introducing more advanced concepts will be provided as well. Therefore, the present Tutorial might act as an introduction and guideline aimed at new prospective users of these techniques as well as a point of reference for more experienced researcher

ON CHARGE CARRIER DENSITY IN ORGANIC SOLAR CELLS OBTAINED VIA CAPACITANCE SPECTROSCOPY

The determination of the voltage-dependent density of free charge carriers via capacitance spectroscopy is considered an important step in the analysis of emerging photovoltaic technologies, such as organic and perovskite solar cells. In particular, an intimate knowledge of the density of free charge carriers is required for the determination of crucial parameters such as the effective mobility, charge carrier lifetime, nongeminate recombination coefficients, average extraction times, and competition factors. Hence, it is paramount to verify the validity of the commonly employed approaches to obtain the density of free charge carriers. The advantages, drawbacks, and limitations of the most common approaches are investigated in detail and strategies to mitigate misleading values are explored. To this end, two types of nonfullerene organic solar cells based on a PTB7-Th:ITIC-2F blend and a PM6:Y6 blend, respectively, are used as a case study to assess how subsequent analyses of the nongeminate recombination dynamics depend on the chosen approach to calculate the density of free charge carriers via capacitance spectroscopy

A Review on the Materials Science and Device Physics of Semitransparent Organic Photovoltaics

In this review, the current state of materials science and the device physics of semitransparent organic solar cells is summarized. Relevant synthetic strategies to narrow the band gap of organic semiconducting molecules are outlined, and recent developments in the polymer donor and near-infrared absorbing acceptor materials are discussed. Next, an overview of transparent electrodes is given, including oxides, multi-stacks, thin metal, and solution processed electrodes, as well as considerations that are unique to ST-OPVs. The remainder of this review focuses on the device engineering of ST-OPVs. The figures of merit and the theoretical limitations of ST-OPVs are covered, as well as strategies to improve the light utilization efficiency. Lastly, the importance of creating an in-depth understanding of the device physics of ST-OPVs is emphasized and the existing works that answer fundamental questions about the inherent changes in the optoelectronic processes in transparent devices are presented in a condensed way. This last part outlines the changes that are unique for devices with increased transparency and the resulting implications, serving as a point of reference for the systematic development of next-generation ST-OPVs

EFFECTS OF RECOMBINATION ORDER ON OPEN-CIRCUIT VOLTAGE DECAY MEASUREMENTS OF ORGANIC AND PEROVSKITE SOLAR CELLS

Non-geminate recombination, as one of the most relevant loss mechanisms in organic and perovskite solar cells, deserves special attention in research efforts to further increase device performance. It can be subdivided into first, second, and third order processes, which can be elucidated by the effects that they have on the time-dependent open-circuit voltage decay. In this study, analytical expressions for the open-circuit voltage decay exhibiting one of the aforementioned recombination mechanisms were derived. It was possible to support the analytical models with experimental examples of three different solar cells, each of them dominated either by first (PBDBT:CETIC-4F), second (PM6:Y6), or third (irradiated CH3NH3PbI3) order recombination. Furthermore, a simple approach to estimate the dominant recombination process was also introduced and tested on these examples. Moreover, limitations of the analytical models and the measurement technique itself were discussed. View Full-Text Keywords: organic solar cells; perovskite solar cells; non-geminate recombination; recombination order; open-circuit voltage deca

On Charge Carrier Density in Organic Solar Cells Obtained via Capacitance Spectroscopy

The determination of the voltage-dependent density of free charge carriers via capacitance spectroscopy is considered an important step in the analysis of emerging photovoltaic technologies, such as organic and perovskite solar cells. In particular, an intimate knowledge of the density of free charge carriers is required for the determination of crucial parameters such as the effective mobility, charge carrier lifetime, nongeminate recombination coefficients, average extraction times, and competition factors. Hence, it is paramount to verify the validity of the commonly employed approaches to obtain the density of free charge carriers. The advantages, drawbacks, and limitations of the most common approaches are investigated in detail and strategies to mitigate misleading values are explored. To this end, two types of nonfullerene organic solar cells based on a PTB7-Th:ITIC-2F blend and a PM6:Y6 blend, respectively, are used as a case study to assess how subsequent analyses of the nongeminate recombination dynamics depend on the chosen approach to calculate the density of free charge carriers via capacitance spectroscopy

THE EFFECTS OF DIFFERENT ELECTRON TRANSPORT LAYER MATERIALS ON THE PHOTOVOLTAIC PROPERTIES OF FLEXIBLE AND PRINTED PEROVSKITE SOLAR CELLS

This work is dedicated to fabricate PSCs on the surface of flexible polyethylene terephthalate with a layer of transparent conducting indium-tin-oxide (PET/ITO) using a slot-die coating method and studying the effect of different electron-transporting layers materials on the performance of FPSCs

OPTICAL AND ELECTRICAL PROPERTIES OF GRAPHITE THIN FILMS PREPARED BY DIFFERENT METHODS

The paper reports on the structural, optical and electrical properties of graphite thin films prepared by two methods: the vacuum-free method "Pencil-on-semiconductor" and via the electron beam evaporation. Graphite thin films prepared by the non-vacuum method has annealed at a temperature of 920K.The transmission spectra of the investigated graphite films and the electrical properties of these thin films were measured at T = 300 K. The value of the height of barriers Eb at the grain boundaries and the temperature dependence of the electrical conductivity in the range ln(σ·T1/2) = f(103/T) were determined, It is established that the height of the barrier at the grain boundaries for the drawn graphite films is Eb = 0.03 eV, for annealed Eb = 0.01 eV and for the graphite films deposited by the electron beam evaporation Eb = 0.04 eV, ie for annealed film the barrier height is the smallest. It is shown that graphite films deposited by the electron beam evaporation reveals the highest transmittance (T550 ≈ 60%), and the transmission of drawn films is the lowest, annealing leads to its increase. The minimum values ​​of transmission at a wavelength λ = 250nm are due to the scattering of light at the defects that are formed at the grain boundaries. Annealed graphite films have been found to possess the best structural perfection because they have the lowest resistivity compared to non-annealed films and electron-beam films and have the lowest barrier height. Simultaneous increase of transmission in the whole spectral range, increase of specific electrical conductivity and decrease of potential barrier at grain boundaries of the annealed drawn graphite film clearly indicate ordering of drawn graphite flakes transferred onto anew substrate, which led to the reduction of light scattering and the improvement of charge transport due to the larger area of ​​overlap between graphite flake

Physical properties of the heterojunction MoOx/n-CdTe as a function of the parameters of CdTe crystals

MoOx/n-CdTe photosensitive heterostructures were prepared by the deposition of molybdenum oxide thin films onto three different n-type CdTe substrates (ρ1=0.4 Ωfirst>cm, ρ2=10 Ωfirst>cm, ρ3=40 Ωfirst>cm) by DC reactive magnetron sputtering. The height of the potential barrier and series resistance of the MoOx/CdTe heterojunctions were investigated. The dominating current transport mechanisms through the heterojunctions were determined at forward and reverse biases

ELECTRICAL AND PHOTOELECTRIC PROPERTIES OF HETEROJUNCTIONS MOOX/N-CD1-XZNXTE

The paper presents the results of studies of the optical and electrical properties of МоOx/n-Cd1-хZnхTe semiconductor heterojunctions made by depositing MoOx films on a pre-polished surface of n-Cd1-хZnхTe plates (5 × 5 × 0.7 mm3) in a universal vacuum installation Leybold - Heraeus L560 using reactive magnetron sputtering of a pure Mo target. Such studies are of great importance for the further development of highly efficient devices based on heterojunctions for electronics and optoelectronics. The fabricated МоOx/n‑Cd1‑хZnхTe heterojunctions have a large potential barrier height at room temperature (φ0 = 1.15 eV), which significantly exceeds the analogous parameter for the МоOx/n-CdTe heterojunction (φ0 = 0.85 eV). The temperature coefficient of the change in the height of the potential barrier was experimentally determined to be d(φ0)/dT = -8.7·10-3 eV K, this parameter is four times greater than the temperature coefficient of change in the height of the potential barrier for MoOx/n-CdTe heterostructures. The greater value of the potential barrier height of the МоOx/n-Cd1-хZnхTe heterojunction is due to the formation of an electric dipole at the heterointerface due to an increase in the concentration of surface states in comparison with MoOx/n-CdTe heterostructures, and this is obviously associated with the presence of zinc atoms in the space charge region and at the metallurgical boundary section of the heteroboundary. In МоOx/n‑Cd1-хZnхTe heterojunctions, the dominant mechanisms of current transfer are generation-recombination and tunneling-recombination with the participation of surface states, tunneling with forward bias, and tunneling with reverse bias. It was found that МоOx/n-Cd1-хZnхTe heterojunctions, which have the following photoelectric parameters: open circuit voltage Voc = 0.3 V, short circuit current Isc = 1.2 mA/cm2, and fill factor FF = 0.33 at an illumination intensity of 80 mW/cm2 are promising for the manufacture of detectors of various types of radiation. The measured and investigated impedance of the МоOx/n-Cd1-хZnхTe heterojunction at various reverse biases, which made it possible to determine the distribution of the density of surface states and the characteristic time of their charge-exchange, which decrease with increasing reverse bias